Questions tagged [electromagnetism]

The classical theory of electric and magnetic fields, both in the static and dynamic case. Also covers general questions about magnets, electric attraction/repulsion etc. Distinct from electrical-engineering.

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21 views

What happens if permanent magnets are placed under electric field?

I understand that ferromagnets have domains that align parallel to the applied magnetic field, in which the spin of the unpaired electrons line up. But I am curious as to what happens to such magnets ...
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Do accelerated moving electrons radiate electromagnetic energy?

Do accelerated moving electrons radiate electromagnetic energy? Generally speaking, the charge should absorb energy instead of radiating energy in the acceleration stage, and release energy in the ...
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What's the physical meaning of the statement that “photons don't have positions”?

It's been mentioned elsewhere on this site that one cannot define a position operator for the one-photon sector of the quantized electromagnetic field, if one requires the position operator have ...
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Looking for physical intuition into the Electromagnetic Tensor:

I have done some work with the electromagnetic tensor and I'm fairly good at manipulating it and using it to transform the Maxwell Equations into tensored forms. Admittedly though, I have no physical ...
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132 views

What is the frequency distribution of the electromagnetic wave emitted by a charge accelerating along a given trajectory?

Charged particles radiate when accelerated.From a few other answers from Physics Stack exchange like What is the frequency of a photon emitted by an accelerated charged particle? and How ...
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Vector potential of a linear current

First of all, I apologize in advance for my English, as I know it is not very good. So thank you for making the effort of understanding me. Well, here is my problem. I am trying to prove that if I ...
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Physical meaning of Maxwell's equations and origin of EM waves

Is it possible to describe the physical meaning of Maxwell's equations and show how they lead to electromagnetic wave, with little involvement of mathematics ?
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Energy in radar equation

In the book Skolnik, Introduction to Radar Systems, ed. 2, pag. 52 (sec. Transmitter Power) the radar equation in its simplest form is initially cited: $$R_{\max}=\left[\frac{P_t G A_e \sigma }{(4 \...
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Formula of the Radius of the Circular Path of a Charged Particle in a Uniform Magnetic Field

A charged particle $q$ enters a uniform magnetic field $\vec{B}$ with velocity $\vec{v}$ making an angle $\theta$ with it. Since the Lorentz force is perpendicular to the velocity, the particle will ...
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Is there a formula showing the relationship between the number of iron cores and the output voltage of a transformer?

I know the relationship between number of turns, current and voltage of a transformer, but is there a formula showing the relationship between the number of iron cores and the output voltage of a ...
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if magnet does not feel lenz force, will there be violation of energy conservation?

Consider the following, a magnet and a charge is attached to a platform, which is constrained to move only up and down direction. Now, if magnet is moved towards charge, due to changing magnetic field,...
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Justification of dropping term in Hamiltonian and expectation Values

While reading Sakurai's Modern QM, I was stuck at the point where he explains the absorption and emission of light quanta in atoms. He proceeds with Hamiltonian: $$H= p^2/2m + e\phi(x) -e/mc A\cdot p$...
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Electromagnetism: What do $B$, $H$ and $M$ field lines look like? [duplicate]

I can't understand the meaning behind this picture For the magnet (upper half of picture), it is said that $\oint$$\vec{H}\vec{dl}=0$ because there are no currents in the magnet (non Ampere's ...
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Time travel / Time machine [on hold]

Basically I’m attempting to design a time machine using a new model of physics which relies on the postulate that gravity is not cause by mass or rather that the weight that is supposedly the cause of ...
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Neutrino disbursement anomaly [on hold]

ok a couple yrs back.I posted a theory that states ,terrestrial and deep space neutrino levels have a variable that cant be caused by particulate in the atmosphere. Its caused by the very resonance of ...
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How can a frequency-specific interference in MRI be limited to a single stripe on the corresponding image (zipper artifact)?

The so-called herringbone or spike MRI artifact on a given example could be traced to a specific point(s) in Fourier space ("k-space"). The idea is that during the acquisition of the image, a certain ...
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4answers
269 views

How is it possible for the induced emf to take negative values in Faraday's Law of induction?

Faraday's Law of induction states that the work done per unit charge by the (induced) electric force along a loop of wire, or the emf, is minus the rate of change of magnetic flux through the loop/...
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2answers
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Is the Poynting Vector Continuous

Helmholtz equation for the magnetic field is $$∇^2 \mathbf{H} + k^2 \mathbf{H} = \mathbf{0}$$ Assuming time-harmonic waves Ampere's law can be rearranged $$\mathbf{E} = \frac{∇\times\mathbf{H}}{σ+...
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What is an induced electric field?

I have read in many books about induced current in a coil (Faraday's law), and also the motional emf across a moving conductor in a magnetic field. But somewhere I read about induced electric field ...
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122 views

How can this integration by parts from Dirac's Lectures on Quantum Mechanics be justified?

In Dirac's Lectures on Quantum Mechanics, when he is deriving the Hamiltonian for the free electromagnetic field, he performs an integration by parts: \begin{align*} H &= -L + \int \pi^\mu A_{\mu,...
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1answer
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How is the electromagnetic/gravitational force transmitted? [duplicate]

So I was thinking about how a positive and a negative charge (or positive/positive, negative/negative) interact. I have read previously about how photons carry the electromagnetic force. However, how ...
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How is Electromagnetic segmented-capacitor propulsion system even possible?

The alleged invention - patent US10006446B2 - suggest allowing the creation of lifting force (4) on electrically charged rotating plate when the magnetic flux is turned on and directed towards the ...
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332 views

Vector potential of a solenoid in the Coulomb gauge

I understand the usual argument for calculating the vector potential outside of a solenoid of radius $R$ with $n$ turns per unit length carrying current $I_0$ using $$ \oint \mathbf{A} \cdot d \mathbf{...
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If a rectangular loop is present between two wires carrying current in same direction but fluctuating, what will be the direction of current in loop

The current in $I_1$ is continuously decreasing and the current in $I_2$ is continuously increasing, then what will be the direction of current in the loop? Here's what I know, the wires carrying ...
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Electrons glow by themselves

Electron beams moving at variable speeds emit electromagnetic radiation. In a vacuum tube, an electron beam bent by a strong magnetic field in a circular motion emits light. The luminescence does not ...
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136 views

Electrical work and Coulomb's law

In some text books I've encountered the calculation of electrical work done on a point charge moving in an electric field (e.g. when demonstrating that electric force is conservative) is done by ...
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2answers
119 views

Where does the magnetic field energy of a charged particle moving with a uniform velocity come from?

Consider a charged particle initially at rest with respect to an inertial frame. Let a force act on it so that it gains a velocity 'v'. It now produces a magnetic field that has some energy ...
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2answers
132 views

Electric charge attraction, unlike magnet attraction, can be neutralized, right?

If I put a magnet in a box of nails, it attracts lots of nails. If I put a hydrogen atom missing its electron (a hydron) near a bunch of electrons, it attracts exactly one electron and then it stops ...
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71 views

Why do we have redundant degrees of freedom?

Preliminaries: Consider the homogenous Maxwell's equations $$\partial_\mu F^{\mu\nu}=0.$$ and $$\partial_{\sigma} F_{\mu \nu}+\partial_{\mu} F_{\nu \sigma}+\partial_{\nu} F_{\sigma \mu}=0$$ Since ...
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Velocity of Object from electromagnetic field

I am wondering if someone could provide me with a formula that would tell me at what velocity a projectile can be launched from something using an electromagnetic field. The idea is much like a rail ...
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Triple infinite summation of a 3D Fourier series for Madelung Potential

I'm trying to evaluate the equation below excluding the case when $n_x=n_y=n_z=0$. I know this equation converges everywhere except where x,y, and z are all multiples of $2\pi$. I've attempted ...
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2answers
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Why are there many turns in the primary coil of a step-up electricity transformer?

Since the voltage in a coil is proportional to the number of turns, the ratio of voltages is equal to the ratio of turns. So if you need a big increase in voltage, what stops you from having one turn ...
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2answers
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Prerequisites for Griffiths electrodynamics [duplicate]

I want to start reading Griffiths electrodynamics during my holidays. But I am not sure of the math I need to know to start reading and understand the text thoroughly. So any advice would be ...
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1answer
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Moving charge to define magnetic field

1-why charged particle has to be in motion to define magnetic field? 2-will magnetic force exert any force on a static charge?(as in the static test charge in electric field) 3-A line from my book ...
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1answer
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Thinking of the Faraday field-strength tensor as a 2-form

Background I'm familiar enough with the Faraday tensor $F_{\alpha\beta}$ to know that it's is a 2-form. Hence, at each point $P$ in spacetime $V$, it's a multilinear map $$F: T_PV\times T_pV\to\mathbb{...
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3answers
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Analogy between fluid dynamics and electromagnetism

I've been reading a set of notes (Chapter 13 of Caltech Ph 136 notes from 2004, by Blandford and Thorne) that draw an analogy between fluid dynamics and electromagnetism, identifying the magnetic ...
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Would fast oscillating charged object emit light?

If I were able to mechanically oscillate an electrically charged object at frequencies of 430-580 THz, would the object emit light?
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Galfenol and internal pressure

Galfenol is cool stuff. It’s iron doped with gallium. When under several gigapascals of force it is able to convert applied pressure to electricity at 70% efficiency when under magnetic field. My ...
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Dipole moments in QFT

We say that an electron has a dipole moment (let's arbitrarily focus on magnetic dipole moment), which we can calculate classically and also add quantum corrections. Suppose we measure the electron's ...
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Repulsion b/w two flat cylindrical magnets

Given 2 identical 'flat' cylindrical permanent magnets, $A_1$ and $A_2$. The radius, height and weight of each magnet is denoted by $R$, $H$ and $W$ respectively. (magnets are flat implies that $R$ > $...
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2answers
269 views

Equivalence between Lorenz gauge and continuity equation

I want to show that the Lorenz gauge condition$$ \nabla\cdot \mathbf{A} + \frac{1}{c^2}\frac{\partial\Phi}{\partial t}~~=~~0 \,,$$where $\mathbf{A}$ and $\Phi$ are the vector and scalar potential of ...
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2answers
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How does alternating current provide energy? [duplicate]

In my head, direct current makes complete sense; the electrons carry energy around the circuit to something being powered losing its potential and then return to the battery or whatnot to have their ...
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1answer
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Can electricity be generated using a rotating electromagnet and fixed metal plates enclosing it (as opposed to rotating permanent magnet)?

I was wondering , if this be possible , can we make an adjustable electricty generator? If not, why so? Appreciate any responses .
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Intuition of Maxwell's Equations [duplicate]

Is there an intuitive explanation for Maxwell's equations? I know they are axioms but is there a logical understanding of why instead of mathematical. Both forms don't explicate the scientific ...
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Criteria for electrostatic approximation in microscopic/kinetic plasmas

I started pondering about the electrostatic approximation in relation to plasmas, where a particle's motion $\mathbf x(t)$ is governed by the Lorentz force: $$ m\ddot{\mathbf x}=q(\mathbf E+\dot{\...
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Chemistry without electrons?

This question on the behavior of a universe with massless electrons got me wondering: what would atoms look like if electrons were not merely massless, but rather the lepton fields just didn't exist ...
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1answer
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$B$ field inside a Magnet

From Lorentz Force law there are experimental proofs for defining the direction of the magnetic field $B$ outside the magnet. But when the interior of magnetic material is concerned how is the ...
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1answer
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Effect of a current carrying wire on a point charge

Given a point charge $q$ near a wire carrying current as shown, what would be the effect of the magnetic field produced by the wire on this point charge $q$? I think the sign on $q$ is important so ...
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How do the electric or magnetic fields contain momentum?

I have recently come to know that the electric and magnetic field contain both linear and angular momenta, which are known functions of the electric and magnetic fields at any given point in space and ...
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How is magnetic energy supplied?

Enthusiastic layperson here, curious about magnetism: Scenario 1: Imagine that I lift a piece of iron with my arm from a tabletop a fixed distance upwards, then place it back on the table. In ...